Any Magicband linked to your My Disney Experience account will be active (unless you manually go in and de-activate them). Any Active MagicBand linked to your
MDE is usable for any trip past, present or future (not just the one you ordered them for). If you have 3 stays, and order new bands for each stay, you could use any band on any stay. Or you could just order bands for one stay, and decline for the other two, and use the same bands. Your choice. Oh, and you could use MagicBands (free solid color, or purchased graphic bands) that you have from a previous trip if you'd like. So like I used my 2016 Flower & Garden band this summer for our PO FQ stay.
And of course your reservations (including tickets, dining plan and resort stay) are linked to your MDE, so any linked/active bands work for all of that.
As for using past bands on a future trip... remember there are two RF ID transmitters in the bands... the short range (used where you tap the band, like park entry, fastpass, photopass, using dining credits if you have the meal plan, opening resort room door for on-site stay, etc) will continue to work "forever" (or at least until Disney changes technology). The LONG RANGE (used for things like to auto link ride photo/video for Seven Dwarfs Mine Train, locating you with your food at BoG breakfast/lunch, your name on exit of Its a Small World, etc) relies on a battery. That battery is said to only last for two years. So When the battery is dead, short range stuff still works, but long range doesn't. With the Magicband 2.0 bands, you can remove/replace the ICON (i.e. Mickey head part wtih the RF Transmitters), so you can request similar solid color band for your trip, and put its icon into the old graphic band (if the battery on the old band is dead).
As for dining... I would double check with Disney or your
Travel Agent because my understanding (and experience) is that Dining Credits expire Midnight on check out day.
So for example If you have a single 9 night stay with the standard
Disney Dining Plan, you'd have 18 snacks, 9 QS and 9 TS per person to use anytime throughout your 9 night trip. You could use them all on day 9 if you wanted. BUT if you have 3 stays of 3 nights each, even if back to back, then you have 6 SN, 3 QS and 3 TS to be used on EACH 3 night stay... i.e. they wont carry over and be able to be used later in the trip like credits if you had a single 9 night stay. You must use the first 6SN/3QS/3TS during the first stay, 6SN/3QS/3TS during stay #2, etc. If you don't use them before check out of that stay, they expire. You can't "save" credits from the 1st stay to use on the next stay and so on. Further, your Refillable resort mug will work only in the resort where they were activated (Poly mug works at Poly, you'll get new mugs for AK and Boardwalk when you check-in for those).
Again, that is my understanding (and what I've seen on split stays), you might want to double check with Disney to confirm BEFORE you travel to be sure you are using your credits properly.